1. Field of the Invention
The present invention relates to a silicon carbide semiconductor device that uses a silicon carbide substrate and a manufacturing method of the silicon carbide semiconductor device; and particularly relates to a silicon carbide semiconductor device that can reduce interface state density and a manufacturing method of the silicon carbide semiconductor device.
2. Description of the Related Art
Research and development of next generation semiconductor devices that use a silicon carbide (SiC) substrate are advancing. Although silicon carbide can form an insulating film by thermal oxidation like silicon, silicon carbide has a characteristic in that differences in channel mobility near the junction interface (hereinafter, metal oxide semiconductor (MOS) interface) of the silicon carbide substrate and the gate insulating film forming a MOS gate occur depending on the crystal plane, oxidation method, etc. Oxidation methods of silicon carbide include dry oxidation that uses dry oxygen (O2) as the oxidizing species and wet oxidation that uses water vapor (H2O) as the oxidizing species.
Wet oxidation of a (000-1) surface or a (11-20) surface of a silicon carbide substrate has been shown to have high channel mobility compared to that of a (0001) surface. Interface state density is used as an index to evaluate channel mobility by an alternative means. In general, it is known that the smaller the interface state density is at the MOS gate interface, the greater the channel mobility tends to be.
Related to a manufacturing method of a semiconductor device employing such a silicon carbide substrate, a method of improving a thermally oxidized film of SiC on a silicon carbide substrate has been disclosed that reduces flat-band shifting and hysteresis, and includes a process of annealing by oxygen and a process of annealing by an inert gas, after thermal oxidation of the silicon carbide substrate in oxygen or humidified oxygen (for example, refer to Japanese Laid-Open Patent Publication No. H9-199497).
Further, a method of reducing interface state density by heat treatment in an atmosphere that includes hydrogen (H2) gas has been disclosed, where the heat treatment is performed after oxidizing a (000-1) surface of a silicon carbide substrate in a wet atmosphere of H2O gas and oxygen gas, or H2O gas, oxygen gas and an inert gas (for example, refer to Japanese Patent No. 4374437). Although methods of generating the water vapor (H2O) in the wet oxidation include heating pure water, bubbling of pure water by oxygen gas, and the like, at present, pyrogenic schemes using the combustion reaction of O2 gas and H2 gas are common. In such a scheme, the flow ratio of O2 gas and H2 gas is typically rich in O2 since H2 gas in excess has the danger of being explosive. Therefore, the oxidizing atmosphere is an atmosphere of H2O+O2 of generated H2O and unreacted O2.
A method of using a platinum catalyst to increase the reactivity of hydrogen and oxygen, causing the reaction to occur at a temperature lower than the ignition temperature of hydrogen mixed gas and generate moisture without high temperature combustion has been disclosed (for example, refer to Japanese Laid-Open Patent Publication No. 2000-72405. In this method, a flow ratio rich in H2 can be used since there is no risk of explosion consequent to the H2 being in excess.
Subjecting a (000-1) surface or a (11-20) surface of a silicon carbide substrate to wet oxidation in gas that includes H2O and O2, and performing thermal treatment in an atmosphere that includes H2 as hydrogen post oxidation annealing (POA) to reduce the interface state density is said to be for terminating unsaturated bonds (dangling bonds) of silicon atoms on the silicon carbide substrate surface where hydrogen or hydroxyl groups form the interface state.
The interface state density in a case where a (000-1) surface or a (11-20) surface of a silicon carbide substrate is subject to dry oxidation in an atmosphere of only dry oxygen is extremely high and has poor MOS interface characteristics. Further, if hydrogen POA is performed after the dry oxidation, the interface state density is beyond the interface characteristics of wet oxidation and hydrogen POA, which are to reduce interface state density.
Therefore, in gate oxidation for forming a gate insulating film on a (000-1) surface or a (11-20) surface of a silicon carbide substrate, the use of O2 to reduce the interface state density is not effective, whereas the use of H2O and H2 is effective.
Thus, preferably, hydrogen or hydroxyl groups introduced by gate oxidation and POA are segregated in a small region that includes the MOS interface because unsaturated bonds of silicon atoms on the silicon carbide substrate surface forming the interface state will be efficiently terminated and because hydrogen or hydroxyl groups present in the gate insulating film cause electron traps.
In light of the problems above, one object of the present invention is to effectively reduce the interface state density of a (000-1) surface or a (11-20) surface of a silicon carbide semiconductor.